The only success I ever had was with a single AKG 414/ULS in figure eight mode (in a small live space) about eighteen inches from the strings, towards the treble side. The 'null' was actually pointed at the base of the harp, it sounded very ethereal.
How is it being used in the mix? As with any classical string instrument, it really depends on if it is used to be an upfront soloist, or an ambient 'room' instrument. Also depends on the context in which is sits...what type of music?
If it's more of a traditional classical 'room' sound, I'd go with stereo ribbons out front and overhead.
If it's a modern soloist sound, a spot mic would work best...I'd still try to work with a ribbon, but a LD FET Condenser would work well also.
I'd think that there would be potential noise/gain issues with using ribbons on a harp....it's just too low volume instrument and will require lots of gain from the preamp. I think I'd be more inclined to use a large diaphragm condensor.